David Radice

A large-scale dynamo and magnetoturbulence in rapidly rotating core-collapse supernovae

Magnetohydrodynamic turbulence is important in many high-energy astrophysical systems, where instabilities can amplify the local magnetic field over very short timescales. Specifically, the magnetorotational instability and dynamo action have been suggested as a mechanism for the growth of magnetar-strength magnetic fields (of 1015 gauss and above) and for powering the explosion of a rotating massive star. Such stars are candidate progenitors of type Ic-bl hypernovae, which make up all supernovae that are connected to long γ-ray bursts. The magnetorotational instability has been studied with local high-resolution shearing-box simulations in three dimensions, and with global two-dimensional simulations, but it is not known whether turbulence driven by this instability can result in the creation of a large-scale, ordered and dynamically relevant field. Here we report results from global, three-dimensional, general-relativistic magnetohydrodynamic turbulence simulations. We show that hydromagnetic turbulence in rapidly rotating protoneutron stars produces an inverse cascade of energy. We find a large-scale, ordered toroidal field that is consistent with the formation of bipolar magnetorotationally driven outflows. Our results demonstrate that rapidly rotating massive stars are plausible progenitors for both type Ic-bl supernovae and long γ-ray bursts, and provide a viable mechanism for the formation of magnetars. Moreover, our findings suggest that rapidly rotating massive stars might lie behind potentially magnetar-powered superluminous supernovae.

Dynamical mass ejection from binary neutron star mergers

We present fully general-relativistic simulations of binary neutron star mergers with a temperature and composition dependent nuclear equation of state. We study the dynamical mass ejection from both quasi-circular and dynamical-capture eccentric mergers. We systematically vary the level of our treatment of the microphysics to isolate the effects of neutrino cooling and heating and we compute the nucleosynthetic yields of the ejecta. We find that eccentric binaries can eject significantly more material than quasi-circular binaries and generate bright infrared and radio emission. In all our simulations the outflow is composed of a combination of tidally- and shock-driven ejecta, mostly distributed over a broad ∼60∘ angle from the orbital plane, and, to a lesser extent, by thermally driven winds at high latitudes. Ejecta from eccentric mergers are typically more neutron rich than those of quasi-circular mergers. We find neutrino cooling and heating to affect, quantitatively and qualitatively, composition, morphology, and total mass of the outflows. This is also reflected in the infrared and radio signatures of the binary. The final nucleosynthetic yields of the ejecta are robust and insensitive to input physics or merger type in the regions of the second and third r-process peaks. The yields for elements on the first peak vary between our simulations, but none of our models is able to explain the Solar abundances of first-peak elements without invoking additional first-peak contributions from either neutrino and viscously-driven winds operating on longer timescales after the mergers, or from core-collapse supernovae.

Discontinuous Galerkin methods for general-relativistic hydrodynamics: formulation and application to spherically symmetric spacetimes

We have developed the formalism necessary to employ the discontinuous-Galerkin approach in generalrelativistic hydrodynamics. The formalism is first presented in a general four-dimensional setting and then specialized to the case of spherical symmetry within a 3 þ 1 splitting of spacetime. As a direct application, we have constructed a one-dimensional code, EDGES, which has been used to assess the viability of these methods via a series of tests involving highly relativistic flows in strong gravity. Our results show that discontinuous-Galerkin methods are able not only to handle strong relativistic shock waves but, at the same time, to attain very high orders of accuracy and exponential convergence rates in smooth regions of the flow. Given these promising prospects and their affinity with a pseudospectral solution of the Einstein equations, discontinuous-Galerkin methods could represent a new paradigm for the accurate numerical modelling in relativistic astrophysics.

GW170817: Joint Constraint on the Neutron Star Equation of State from Multimessenger Observations

Gravitational waves detected from the binary neutron star (NS) merger GW170817 constrained the NS equation of state by placing an upper bound on certain parameters describing the binarys tidal interactions. We show that the interpretation of the UV/optical/infrared counterpart of GW170817 with kilonova models, combined with new numerical relativity results, imply a complementary lower bound on the tidal deformability parameter. The joint constraints tentatively rule out both extremely stiff and soft NS equations of state.

NEUTRINO-DRIVEN CONVECTION IN CORE-COLLAPSE SUPERNOVAE: HIGH-RESOLUTION SIMULATIONS

We present results from high-resolution semi-global simulations of neutrino-driven convection in core-collapse supernovae. We employ an idealized setup with parametrized neutrino heating/cooling and nuclear dissociation at the shock front. We study the internal dynamics of neutrino-driven convection and its role in re-distributing energy and momentum through the gain region. We find that even if buoyant plumes are able to locally transfer heat up to the shock, convection is not able to create a net positive energy flux and overcome the downwards transport of energy from the accretion flow. Turbulent convection does, however, provide a significant effective pressure support to the accretion flow as it favors the accumulation of energy, mass and momentum in the gain region. We derive an approximate equation that is able to explain and predict the shock evolution in terms of integrals of quantities such as the turbulent pressure in the gain region or the effects of non-radial motion of the fluid. We use this relation as a way to quantify the role of turbulence in the dynamics of the accretion shock. Finally, we investigate the effects of grid resolution, which we change by a factor 20 between the lowest and highest resolution. Our results show that the shallow slopes of the turbulent kinetic energy spectra reported in previous studies are a numerical artefact. Kolmogorov scaling is progressively recovered as the resolution is increased.

Implicit large eddy simulations of anisotropic weakly compressible turbulence with application to core-collapse supernovae

In the implicit large eddy simulation (ILES) paradigm, the dissipative nature of high-resolution shock-capturing schemes is exploited to provide an implicit model of turbulence. The ILES approach has been applied to different contexts, with varying degrees of success. It is the de-facto standard in many astrophysical simulations and in particular in studies of core-collapse supernovae (CCSN). Recent 3D simulations suggest that turbulence might play a crucial role in core-collapse supernova explosions, however the fidelity with which turbulence is simulated in these studies is unclear. Especially considering that the accuracy of ILES for the regime of interest in CCSN, weakly compressible and strongly anisotropic, has not been systematically assessed before. Anisotropy, in particular, could impact the dissipative properties of the flow and enhance the turbulent pressure in the radial direction, favouring the explosion. In this paper we assess the accuracy of ILES using numerical methods most commonly employed in computational astrophysics by means of a number of local simulations of driven, weakly compressible, anisotropic turbulence. Our simulations employ several different methods and span a wide range of resolutions. We report a detailed analysis of the way in which the turbulent cascade is influenced by the numerics. Our results suggest that anisotropy and compressibility in CCSN turbulence have little effect on the turbulent kinetic energy spectrum and a Kolmogorov k−5/3

High-Order Fully General-Relativistic Hydrodynamics: new Approaches and Tests

k^{-5/3}

Electron-capture and Low-mass Iron-core-collapse Supernovae: New Neutrino-radiation-hydrodynamics Simulations

scaling is obtained in the inertial range. We find that, on the one hand, the kinetic energy dissipation rate at large scales is correctly captured even at low resolutions, suggesting that very high “effective Reynolds number” can be achieved at the largest scales of the simulation. On the other hand, the dynamics at intermediate scales appears to be completely dominated by the so-called bottleneck effect, i.e., the pile up of kinetic energy close to the dissipation range due to the partial suppression of the energy cascade by numerical viscosity. An inertial range is not recovered until the point where high resolution ∼5123, which would be difficult to realize in global simulations, is reached. We discuss the consequences for CCSN simulations.

How loud are neutron star mergers

We present a new approach for achieving high-order convergence in fully general-relativistic hydrodynamic simulations. The approach is implemented in WhiskyTHC, a new code that makes use of state-of-the-art numerical schemes and was key in achieving, for the first time, higher than second-order convergence in the calculation of the gravitational radiation from inspiraling binary neutron stars (Radice et al 2014 Mon. Not. R. Astron. Soc. 437 L46– L50). Here, we give a detailed description of the algorithms employed and present results obtained for a series of classical tests involving isolated neutron stars. In addition, using the gravitational-wave emission from the late-inspiral and merger of binary neutron stars, we make a detailed comparison between the results obtained with the new code and those obtained when using standard second-order schemes commonly employed formatter simulations in numerical relativity. We find that even at moderate resolutions and for binaries with large compactness, the phase accuracy is improved by a factor 50 or more.

Shock-turbulence interaction in core-collapse supernovae

We present new 1D (spherical) and 2D (axisymmetric) simulations of electron-capture (EC) and low-mass iron-core-collapse supernovae (SN). We consider six progenitor models: the ECSN progenitor from Nomoto (1984, 1987); two ECSN-like low-mass low-metallicity iron core progenitors from Heger (private communication); and the 9-, 10-, and 11-